Wet capacitor packs are usually enclosed in pressure-tight containers, often in the form of cases. With regard to the fact that excess pressure can occur in the capacitor, there must be som kind of safety device which can open communication between the interior of the container and the surrounding atmosphere so that excess pressure may be vented. Up to now, such safety devices have usually consisted of a rubber plug disposed in an opening in the container, and dimensioned to yield when an excess pressure of a predetermined value occurs. Instead of a rubber plug a diaphragm has sometimes been used, which is allowed to rupture on reaching the excess pressure. Common to both these types of safety device is that the capacitor must be rejected after the excess pressure has occured, since the open communication to the surrounding atmosphere remains even after reduction in pressure.
It is also known to vent the excess pressure via a lift valve arranged in the wall or lid of the container, the lift valve containing an elastic valve body, e.g. of rubber. Due to its elasticity, the body is compressed by, and opens for the excess pressure, subsequently closing again when the excess pressure has been dissipated. Even though this type of safty device automatically vents the excess pressure without the need of service or exchange after the excess pressure has occured, it has a short life, due to ageing phenomena in the elastic valve body, particularly at high temperatures.